Thermal activation of vortex motion in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films at low temperatures

Thermally activated flux vortex motion in a constant applied field is studied at temperatures between 2.5 and 21 K for in situ thin films of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The magnitude and temperature dependence of the dissipation is shown to be consistent with an activation energy U(J), which is independent of temperature in this temperature region, but which is assumed to be a function only of current J. Activation energies U(J) are deduced for currents between 1×${10}^7$ and 2.5×${10}^7$ A/${\mathrm{cm}}^2$ for the samples measured, by considering the change in the decay rate, at constant current, as a function of temperature. These results indicate that the fundamental mechanism for dissipation in this temperature range is thermally activated.